Euro transonic, cryogenic wind tunnel in full swing

In Cologne, Germany, the $412 million European Transonic Wind Tunnel (ETW)– a multinational joint venture among France, Germany, the Netherlands and the UK–is gradually increasing its workload. The main feature of the wind tunnel is the use of cold nitrogen to aerodynamically offset the smaller size of the model tested, compared to the actual aircraft. NASA operates the only other cryogenic wind tunnel in the world, but the ETW has already attracted North American customers, including Bombardier and Boeing.

The first customer test took place in 1995, though it took some time for the wind tunnel to become fully operational. Lionel Baranes, the company’s managing director, said the ETW has been fully operational since the early 2000s. During a July visit organized by the French association of aerospace journalists, he said that last year the ETW used 60 percent of its 2,400-hour capacity. This year, it expects to use between 70 and 75 percent.

The Reynolds number–the ratio between inertia forces and viscosity around the object in the fluid flow–provides a measurement of how realistic wind tunnel tests are. Using a scaled-down model of the airplane decreases the numerator. The idea is to decrease the denominator by reducing the fluid’s viscosity. This is possible in conditions of low temperature or higher pressure.

Setbacks during the development of the C-5 airlifter illustrate the importance of the Reynolds number. In 1969, wind-tunnel testing prompted Lockheed engineers to adopt a wing profile with 12-percent thickness. But flight tests showed that 14 percent was possible. Fuel capacity was therefore 16 percent less than it could have been. There was also some impact on the structural lifetime. Some 77 aircraft were later modified at considerable expense, Baranes recalled.

The ETW takes into account the importance of the Reynolds number. The wind tunnel can blow nitrogen gas at temperatures as low as -163 degrees C. Pressure can reach 4.5 bar. Whereas conventional wind tunnels can provide Reynolds numbers of 10 million, the ETW has a Reynolds range of 50 to 80 million, which is consistent with that of an actual airliner in flight. The half-span model of the behemoth Airbus A380 approaches ETW’s limits, however.

This enhanced level of realism calls for the use of a high-quality model. “The final polish of the metal surface must be 50 times finer than that of the actual aircraft,” test engineer Eric Germain said. The cost of a model can thus be close to $600,000. The alloy used is called maraging steel, and it keeps deformation reasonable, despite cold and aerodynamic forces.

Temperature, pressure and Mach number (between 0.5 and 1.3) can vary independently. “In conventional wind tunnels, you get secondary aeroelastic effects because you have to vary the pressure along with the Reynolds number,” Germain explained. The model sits in a 33-foot-long, 8-foot-wide and 6.6-foot-high test section. The power of the motor that blows nitrogen gas on to the model is 50 megawatts.

The company counts Airbus and Dassault among its customers. Dassault used the wind tunnel to optimize the Falcon 7X’s wing and in the future plans to use the ETW earlier in its business jet designs.

Non-European customers made up about half of the ETW’s business last year. In fact, Boeing performed low-speed, high-lift tests in the ETW with a model of the in-development 787. Engineers used the entire pressure and temperature range of the facility. Last year Bombardier conducted the first evaluation of the C-Series at cruise-flight Reynolds number.

Separate customer suites ensure confidentiality at the facility. The model cart is placed into the test section through a movable ceiling. A customer seeking total secrecy could even test a model and retrieve data without ETW employees viewing any of the process.